PERTURBATION-THEORY AND COMPUTER-SIMULATIONS FOR LINEAR AND RING MODEL POLYMERS

Theory and computer simulations of model polymers are presented. Polym ers are modeled as freely-jointed beads, with the nonbonded bead-bead interactions given by the Lennard-Jones potential; a harmonic spring p otential is used for the bonding interactions. Simulation results for linear chains containing 200 beads are presented. A thermodynamic pert urbation theory for polymerization is compared to simulation data for chains containing from two to 200 beads, over a range of temperatures and densities. Two variations of the theory are investigated, one util izing a reference fluid of monomers (TPT1-M), and another employing a dimer reference fluid (TPT1-D). It is found that TPT1-D is far more ac curate for predicting the pressures of linear flexible chains than TPT 1-M. At low densities TPT1-M predicts internal energies that are too h igh compared to simulation data. This is because TPT1-M neglects intra molecular contributions to the configurational energy. TPT1-D gives a more accurate description of the low density energies of flexible chai ns by incorporating structural information about the dimer fluid into the reference term. Computer simulations of ring polymers are presente d. Noninterlocking flexible rings with 3, 8, and 20 beads are modeled. Simulations of rigid planar rings containing 3 and 8 beads are also p resented. Pressures and energies for rigid and flexible 3-mer rings ar e virtually identical, even though the flexible model includes bond vi brations which are absent in the rigid ring model. In contrast, the pr essure of the rigid 8-mer ring fluid is always higher than the pressur e of flexible ring fluids at the same temperature and density. Extensi ons of TPT1-M and TPT1-D for ring polymers are compared with simulatio n results for flexible and rigid rings. The monomer reference theory p redicts pressures that are too high for flexible rings but too low for rigid 8-mer rings at high densities. TPT1-D for rings gives good agre ement for pressures and energies of flexible rings at high densities, but incorrectly predicts a two-phase region for ring polymers at super critical temperatures. (C) 1996 American Institute of Physics.